Research / Cycle phases & the fertile window
Cycle tracking only means something if the phase boundaries are drawn from real data, not a rule of thumb. Here's what Little Greenhouse's phase logic is actually built on.
Estrogen rises through the follicular phase toward ovulation; progesterone dominates the luteal phase, with both dropping before menses — the basis for how the app frames each cycle phase.
Defined as roughly the five days before ovulation through the day of ovulation itself, based on the foundational study tracking real conception timing against ovulation.
A March 2026 preprint describes a model that infers which of the four cycle phases someone is in purely from self-reported daily symptoms — the same kind of data a tracking app already collects — validated against real hormone assays (LH, estradiol, progesterone) as ground truth, rather than calendar-day counting alone. This is a preprint and has not yet been peer-reviewed, so it’s noted here as an early, promising direction, not an adopted method.
Sperm can survive in the reproductive tract for up to five days, while the egg is viable for about 24 hours after ovulation — so the fertile window research-backed length reflects sperm survival time, not a rounder number.
No — phase boundaries are calculated from your own logged cycle length and ovulation signals, not a fixed 28-day assumption.
Little Greenhouse is still finishing its way to Google Play.
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